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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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<blockquote data-quote="user 4386" data-source="post: 403377" data-attributes="member: 4386"><p>Spinels are really a huge group and some are iron aluminium oxide, others zinc aluminium oxide etc (green gahnite that you see around Broken Hill) - it is those with aluminium in their formula that form gems (those without aluminium include the common iron mineral magnetite and the main ore of chromium, chromite). The colour differences in the magnesium aluminium oxide spinel that is most common as a gem (and referred to just as spinel) is just minor chromium and iron to give violet to purple, cobalt and iron to give blue, iron to give bluish green, chromium can give pink and dark green, iron bluish-green, chromium pink to red. It is not only the element that determines the colour but where the element occurs in the crystal structure and what other elements occur with it and where they occur in the structure.</p><p></p><p>Black spinel is cut in Australia (eg Rubyvale, Weld River): <a href="https://www.bespoke-gems.com/SacredGeometrics_Gemstones_BlackSpinel.php" target="_blank">https://www.bespoke-gems.com/SacredGeometrics_Gemstones_BlackSpinel.php</a> - it may be the iron aluminium spinel hercynite (less likely the manganese aluminium spinel galaxite). There is a continuous range between the magnesium aluminium and iron aluminium spinels, commonly referred to as pleonaste or ceylonite and like black spinel not particularly attractive (in my opinion).</p><p></p><p>I suspect (speculate) that the reason for us having black spinels with sapphires is that most of our sapphires probably come out of basalts in which iron is abundant (and in which the iron-rich spinels hercynite and magnetite occur). Coloured spinel appears almost absent from Victoria and Tasmania (with which I am familiar) just the wrong geology (the same problem with rubies). Coloured spinels from places like Burma and Sri Lanka occur with rubies and are often in limestones a different geology.</p></blockquote><p></p>
[QUOTE="user 4386, post: 403377, member: 4386"] Spinels are really a huge group and some are iron aluminium oxide, others zinc aluminium oxide etc (green gahnite that you see around Broken Hill) - it is those with aluminium in their formula that form gems (those without aluminium include the common iron mineral magnetite and the main ore of chromium, chromite). The colour differences in the magnesium aluminium oxide spinel that is most common as a gem (and referred to just as spinel) is just minor chromium and iron to give violet to purple, cobalt and iron to give blue, iron to give bluish green, chromium can give pink and dark green, iron bluish-green, chromium pink to red. It is not only the element that determines the colour but where the element occurs in the crystal structure and what other elements occur with it and where they occur in the structure. Black spinel is cut in Australia (eg Rubyvale, Weld River): [url]https://www.bespoke-gems.com/SacredGeometrics_Gemstones_BlackSpinel.php[/url] - it may be the iron aluminium spinel hercynite (less likely the manganese aluminium spinel galaxite). There is a continuous range between the magnesium aluminium and iron aluminium spinels, commonly referred to as pleonaste or ceylonite and like black spinel not particularly attractive (in my opinion). I suspect (speculate) that the reason for us having black spinels with sapphires is that most of our sapphires probably come out of basalts in which iron is abundant (and in which the iron-rich spinels hercynite and magnetite occur). Coloured spinel appears almost absent from Victoria and Tasmania (with which I am familiar) just the wrong geology (the same problem with rubies). Coloured spinels from places like Burma and Sri Lanka occur with rubies and are often in limestones a different geology. [/QUOTE]
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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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